A so-called waveguide tube slot antenna is sometimes used as an antenna for transmitting or receiving a radio wave. As an example of the waveguide tube slot antenna, there is known a waveguide tube slot antenna disclosed in Patent Literature 1. The waveguide tube slot antenna disclosed in Patent Literature 1 is formed by arranging a plurality of slot-like antenna elements (radiating slots) in a cross-sectionally rectangular metallic tube (waveguide tube) that is seamless in its transverse section (cross section orthogonal to a tube axis direction) at predetermined intervals.
The waveguide tube slot antenna may be used as an antenna for transmitting or receiving a radio wave having a high-frequency bandwidth (for example, radio wave having a millimeter wave band) and a radio wave having a low-frequency bandwidth (for example, radio wave having a centimeter wave band). The radio wave having a millimeter wave band is used, for example, for an on-vehicle radar system, and the radio wave having a centimeter wave band is used, for example, for a satellite broadcasting system using a broadcasting satellite (BS), a communication satellite (CS), or the like, a data transmission system such as a wireless LAN or Bluetooth (trademark), and an electronic toll collection system (ETC (trademark)). Note that, the radio wave having a millimeter wave band represents a radio wave having a wavelength of from 1 mm to 10 mm and a frequency of from 30 GHz to 300 GHz, and the radio wave having a centimeter wave band represents a radio wave having a wavelength of from 10 mm to 100 mm and a frequency of 3 GHz to 30 GHz.